The present invention relates to airflow distribution of bleed air extracted from turbine engines, and more particularly to a system for equalizing the airflow distribution among a plurality of engines.
It is a common practice to bleed air from one or more gas turbine aircraft engines to provide pneumatic and thermal power to different aircraft systems, for example, for cabin pressurization and temperature control, equipment environmental control, thrust reversing systems, anti-icing equipment, and pneumatically powered equipment. It is desirable to distribute the burden of supplying air for these auxiliary functions among the several engines of the aircraft. U.S. Pat. No. 4,765,131 entitled AIRCRAFT ENGINE BLEED AIR FLOW BALANCING TECHNIQUE recognizes these problems, measures the pressure drop across a heat exchanger and employs limited authority negative feedback to control a pressure regulating valve for each of two duct systems. Without accurate allocation of the airflow burden among the several engines, the engine having the greatest burden experiences disproportionate diminished fuel economy, elevated operating temperature, and increased wear and maintenance requirements. These problems are discussed in greater detail in the U.S. Pat. No. 4,765,131.
In an application entitled BLEED AIR FLOW REGULATORS WITH FLOW BALANCE, U.S. Pat. No. 5,155,991 recognizes the shortcomings of the U.S. Pat. No. 4,765,131 patented system due to errors in sensing the pressure drop across the heat exchanger and measures not only the pressure differential across a heat exchanger (precooler), but also another downstream pressure differential and precooler outlet temperature and pressure values in an attempt to refine the equalization of the flow of bleed air from each of several gas turbine engines. The U.S. Pat. No. 5,155,991 control scheme for each flow branch requires the flow information from its own branch and the flow information from the other branch. The cross-feeding of flow information is the key to equalize the flows in the two branches. The system is suitable only for a two-branch flow system. A control scheme for a four-engine system is an extremely challenging control problem, with cross-feeding of all four flow information to all four control channels, creating enormous interaction problem. There remains a need for comparatively simple and economical, yet highly accurate and reliable technique for allocating bleed airflow among several aircraft engines, and this need is particularly acute for applications involving more than two engines.
It is desirable to equally share the air supply responsibilities among the several engines of an aircraft, and to do so in an economical and accurate manner.
The present invention provides solutions to the above problems by measuring individual flow rates and a common downstream manifold pressure, and uses electronic/computer means to implement negative-feedback control for creating identical, e.g., linear, pressure-versus-flow characteristics for all flow branches. Flow branches with identical pressure-drop characteristics will allow the supply sources to naturally and accurately flow-share.
The invention comprises, in one form thereof, a technique for allocating an aircraft air supply demand among several independently operable aircraft engines, each supplying air to a common manifold by way of a corresponding air duct having an inlet pressure control valve for regulating the duct airflow. The rate of airflow through each individual duct may be measured and that measurement used in determining the desired air pressure set point. Additionally, a common manifold air pressure is measured. A combination of the determined and measured pressures is used to generate a corrective control command to the corresponding pressure control valve for each duct independently of the others. Airflow as used herein means the air flow rate, e.g. measured in pounds of air per minute, while pressure drop is measured, for example in pounds per square inch.
An advantage of the present invention is that all ducts can be independently set by controlling individual pressure regulating valves to create identical pressure-drop characteristics downstream of the valves as a function of mass flow rate regardless of differences in downstream pressure drops due to the individual duct length, cross-section area, shape and internal contamination, which is indeterminate in time. Ducts with identical pressure-drop characteristics will flow-share naturally.